Diversity and Abundance of Crustacean Zooplankton Community in Gilgel Gibe Reservoir, Southwestern Ethiopian Highland

Aims: The study was conducted to assess spatio-temporal dynamics of diversity and abundance of crustacean zooplankton in Gilgel Gibe Reservoir.

Place and Duration of Study: The reservoir is located in Omo-Gibe catchment in southwestern Ethiopian highland. Samples were collected fortnightly at three major localities representing riverine, transition and lacustrine zones from March to August 2013.

Methodology: Water samples for zooplankton were collected with plankton net and fish were sampled using gillnets. Zooplankton identification and counting were made in laboratory. Chlorophyll-a concentration and major physico-chemical parameters (dissolved oxygen, pH, water temperature, and electric conductivity) were measured in situ using standard meters. Water transparency was measured using Secchi disc. Data were analyzed using two-way analysis of variance (ANOVA). The habitat patterns of crustacean zooplankton were assessed using two way cluster analysis, and association between abundance of crustacean zooplankton and the measured environmental variables was explored using canonical correspondence analysis (CCA).

Results: Forty-nine species and two subspecies of crustacean zooplankton in three major categories were identified. The abundance of crustacean zooplankton varied spatio-temporally (P = 0.01). The crustacean zooplankton diversity was higher during the dry season while abundance was higher during wet season. While most of the crustacean zooplankton species (63%) were distributed virtually across all the sampling sites of the reservoir, a few species turned to be habitat specialists occurring only at a particular site. The dynamics of cyclopoids largely related to chlorophyll-a, water temperature, and electric conductivity while the dynamics of calanoids largely associated with Secchi depth, and partly with amount of dissolved oxygen and water pH. The dynamics of cladocerans remained unaccounted for by environmental variables measured during study period. Fish predation appeared to have largely structured the large sized cladoceran zooplankton as these were the most frequent and highest contributors to Oreochromis niloticus and Labeobarbus intermedius diets, the two dominant fish species in the reservoir.

Conclusion: This indicates that dynamics of the crustacean zooplankton community of the reservoir is regulated by various factors including primary productivity, major physico-chemical parameters and fish predation.